I was wondering about the physics for mechanical ventilation with heat recovery. My understanding is that it’s a passive system that has a head exchanger formed of many thin layers of a metal (presumably aluminium) which then alternate airstreams entering and leaving a house to heat up the incoming air

The diagrams you’ll look at will show an example like it being 0º outside and 20º inside, then after the heat exchange, it heats up the incoming air to 18º

Is this not bad science? The pressure of the house has to remain constant, so the incoming volume of air has to equal the outgoing volume of air. At best - if the air had infinite time to exchange heat, the best you’d achieve is 10º for the incoming air.

In the real case, I’d assume your heat exchanger would reach 10º, and the incoming air would interact with it for at most a few seconds. I just can’t see any real heat transfer happening here

What’s your thoughts? A scam, or something that has actual benefits?

Edit - I’ve left the original post in tact - but I did find an answer. It’s a real phenomena called countercurrent flow/countercurrent heat exchange. It’s very important that the flows are in opposite directions - if they’re not, you’ll just reach the equilibrium temperature. But when they flow in the opposite directions, it is possible exchange nearly all the heat. The phenomena also shows up in nature - ducks have a countercurrent heat exchange system in their legs so the are able to recover heat losses from their feet being in water